Plants which handle liquids, such as breweries, dairies and soft drink producers, use pipe networks to transfer various liquids between locations. There is a need to stop or change the various liquid flows for mixing, cleaning or other reasons. This is often accomplished by the use of many switching valves disposed at strategic locations in the pipe networks. For example, a modern brewery might utilize numerous switching valves operated by a remote controller located, for example, in a control room.
Such valves typically are operated by a valve control unit which activates a solenoid for supplying air to a pneumatic cylinder, the ram of which directly switches the valve. It is common practice to hardwire each valve control unit to the controller. This requires the use of a great deal of material in terms of wire and conduit, and a large amount of skilled labor is required to complete the installation. Moreover, revisions and additions result in significant system downtimes and are also expensive in terms of labor and material.
A recently proposed valve control system uses valve control units carried by the air cylinder mounted on each valve. The various valve control units are connected by a common cable to the controller and the cable includes two conductors for transmitting a D.C. voltage superimposed on which is a modulated carrier frequency containing date from the controller for synchronization and valve switching while the valve control units supply data as to the condition of a contact at the valve control unit which is intended to represent the position of the valves. This is a type of frequency shift coding and is somewhat subject to interference which could cause unintended and erroneous valve operation. Furthermore, each valve control unit is required to include electronic circuitry for separating the D.C. voltage, for power, and the modulated carrier frequency for use with encoders, decoders and synchronizers. Reference may be made to U.S. Pat. No. 4,217,647 for a further description of the structure and operation of this system.